WINDSOR UNIVERSITY SCHOOL OF MEDICINE

1. WINDSOR UNIVERSITYSCHOOL OF MEDICINE Dr.Vishal Surender.MD. The Movement of Water Across the Plasma Membrane

2. The Movement of Water Across the Plasma Membrane • Water can move rapidly in and out of cells. • But the partition coefficient of water into lipids is low meaning the permeability of the membrane lipid bilayer for water is low. • Specific membrane proteins that function as water channels explain the rapid movement of water across the plasma membrane • These water channels are small integral membrane proteins known as aquaporins

3. Water Movement NaCl 0 mOsm NaCl 100 mOsm [water] HIGH [water] LOW Aquaporin If membrane impermeable to NaCl

4. CLINICAL CORRELATION- • In the kidney, aquaporin-2 (AQP2) is abundant in the collecting duct and is the target of the hormone vasopressin, also known as antidiuretic hormone. This hormone increases water transport in the collecting duct by stimulating the insertion of AQP2 proteins into the apical plasma membrane. Several studies have shown that AQP2 has a critical role in inherited and acquired disorders of water reabsorptionby the kidney. • For example, diabetes insipidusis a condition in which the kidney loses its ability to reabsorb water properly, resulting in excessive loss of water and excretion of a large volume of very dilute urine (polyuria). Although inherited forms of diabetes insipidus are relatively rare, it can develop in patients receiving chronic lithium therapy for psychiatric disorders, giving rise to the term lithium-induced polyuria. • Both of these conditions are associated with a decrease in the number of AQP2 proteins in the collecting ducts of the kidney.

6. The Movement of Water Across the Plasma Membrane Is Driven by Differences in Osmotic Pressure • osmotic pressure of a solution is defined as the pressure necessary to stop the net movement of water across a selectively permeable membrane • When a membrane separates two solutions of different osmotic pressure, water will move from – • the solution with low osmotic pressure (high water and low solute concentrations) to • the solution of high osmotic pressure (low water and high solute concentrations).

8. Solutions with the same osmotic pressure are called isosmotic. • A solution is hyperosmotic with respect to another solution if it has a higher osmotic pressure and hypo-osmotic if it has a lower osmotic pressure.

9. The Concept of Tonicity • Tonicity describes a solution, and how that solution affects cell volume. • The tonicity of a solution depends not just on the osmolarity of the solution but also on whether the solutes (particles) in the solution are penetrating or not. Rules for predicting tonicity If the cell has a higher concentration of non-penetrating solutes than the solution, there will be net movement of water into the cell. The cell swells, and by definition that solution is hypotonic.

10. RBC hypotonic solution SWELL

11. RBC isotonic solution NO VOLUME CHANGE

12. RBC hypertonic solution SHRINK

13. Tonicity Tonicity describes the volume change of a cell placed in a solution

14. Oral Rehydration Therapy Is Driven by Solute Transport Oral administration of rehydration solutions has dramatically reduced the mortality resulting from cholera and other diseases that involve excessive losses of water and solutes from the gastrointestinal tract. The main ingredients of rehydration solutions are glucose, NaCl, and water. The glucose and Na+ ions are reabsorbed by SGLT1 and other transporters in the epithelial cells lining the lumen of the small intestine . Deposition of these solutes on the basolateral side of the epithelial cells increases the osmolality in that region compared with the intestinal lumen and drives the osmotic absorption of water. Absorption of glucose, and the obligatory increases in absorption of NaCl and water, helps to compensate for excessive diarrheal losses of salt and water.